The replace-ability argument in Peter Singer' Practical Ethics

Can you sacrifice one being for another simply because the second would be happier?

The happiness of one being cannot be sufficient cause to kill another. Either it is acceptable to kill the being or not, the happiness of others is not a valid concern. The dead being cannot take part in the happiness created after and as a result of it's death and therefore cannot be expected to judge such pleasure/happiness as a valid justification or exchange for that death. Death can be validated be cessation of pain but not by the expectation of future pleasure.

In the case of animals for food, it is not the happiness of the eater, or the happiness of the next animal who will be bred to replace this one that matters. It is a contract, (one which obviously the animal cannot be thought to have properly entered into but that is a different issue...) Exchanging the food shelter and care the animal has received for the life and meat that results. Obviously in the modern industrial agricultural system many animals are not provided with happy and fulfilling lives but this is a failure to fulfill the inherent terms of the exchange and not a justification of ending their lives in order to end their suffering. This is one of the great failures of the modern American food system, that we trade our moral responsibilities to the animals we use for food for lower costs and convenience. It is the responsibility of society to provide our livestock with the most pleasant and comfortable lives the healthiest feed and the greatest freedom possible in the time prior to their deaths. This goes not only for those animals we kill for their flesh but also for those we use for all purposes, eggs, milk, wool etc.

Abortion of the disabled fetus, it is not the replacement fetus that matters it is the removal of pain suffered by the disabled child and by the parents as well as the social cost of caring for the child etc. This of course does not make the decision about whether or not to abort a social decision, it remains a private decision to be reached by the parents of the child along with disinterested and honest medical advisers. The choice to abort a fetus cannot be made contingent on the ability, willingness, or intention to have another child to replace the lost fetus. This would present the potential to force a woman to have a disabled child for which she is unable to provide a replacement, or to have a child she does not wish to have merely to replace one she chose to abort due to disability. Women need to be made free to abort or keep a given fetus for whatever reasons she judges appropriate.